Electrical discharge device



June 5, 1934. E. E. CHARLTON 1,961,314

ELECTRICAL DISCHARGE DEVICE Filed Sept. 26, 1928 Inventor: Ernest E.Char-lton,

by WVLZZW/ Hi5 Attorney.

Patented June 5, 1934 UNITED STATES PATENT OFFICE 1,961,814 ELECTRICAL DISCHARGE DEVICE New York Application September 26, 1928, Serial No; 308,417

'7 Claims.

The present invention relates to electrical discharge devices in which an alkali metal such as caesium is present. In devices of this character the alkali metal is generally introduced into the tube from a metallic pellet containing an alkali compound and a reducing agent, the alkali metal being vaporized upon application of heat to the pellet. In such devices, if no control is exercised over the deposition of the alkali metal withinthe tube, the metal condenses at various desired and undesired points often causing stem leakage and thereby reducing the efficiency of the device.

According to the present invention, I have provided an improved arrangement for controlling the position of the alkali metal in electrical discharge devices whereby the metal is present substantially only at desired points within the discharge device. This result is accomplished by applying a coating of Aquadag, which is a colloidal suspension of graphite in water, either to one of the electrodes or to a metallic plate or rod within the device, the Aquadag coated surface being placed in the device at the point where it is desired to position the alkali metal.

Aquadag readily absorbs alkali metal at room temperature and evolves it by slight heating.

The alkali material is introduced into the discharge device from a suitable composition, such as a mixture of caesium or rubidium chloride and calcium, the latter acting as a reducing agent. The alkali metal is quickly absorbed by the Aquadag and thereafter may be liberated if the tube is heated to or operated at a sufficiently high temperature.

Depending on the character of the discharge device,it may be desirable either to retain the caesium on the Aquadag under all conditions or to liberate it during operation of the device. In some devices such as rectifiers, or three-element vacuum tubes, it will be found desirable to liberate the caesium during operation, while in some other devices, for example, gas-filled lamps, it is desirable to retain the caesium on the Aquadag at all times. When the former devices, viz. the rectifiers and three-element tubes cease to operate and the bulb temperature is lowered the alkali is again absorbed by the Aquadag. Freeing and reabsorption of the alkali metal by the Aquadag may be repeated indefinitely as long as the Aquadag surface is not destroyed.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, will best be understood with reference to the following specification when considered in connection with the accompanying drawing in which Fig. 1 is a view partly in section and partly in elevation of a half wave rectifier embodying the features of my invention; Fig. 2 is a view on elevation, partly broken away, of a three-element electrical discharge device embodying a modified form of my invention, while Figs. 3 and 4 are elevational views partly broken away of additional forms of electrical discharge devices, Fig. 3 representing a neon lamp adapted for use in radio transmission of pictures, while Fig 4; is a form of neon lamp employed as a current and voltage indicator and also for illumination purposes. 7

Referring more particularly to the drawing, I have indicated at 1 an evacuated glass receptacle provided with a stem or press 2 on which a hollow cylindrical metal anode 3 is supported by means of wires 4 and 5, the former of which comprises a leading-in wire whereby a potential may be supplied to the anode. A coiled filamentary wire 6 connected to leading-in wires 7 and 8 constitutes the cathode electrode of the device. The inner surface of the anode 3 is coated with Aquadag. A pellet 10, which comprises a nickel casing enclosing a mixture of caesium chloride and a reducing agent such as calcium, is mounted in the vessel 1. In Fig. 1, I have shown the pellet secured to the outside surface of the anode, it may, however, be secured at any desired point.

The vessel or tube 1 is evacuated at a temperature of about 300 350 C. by a suitable pump. While connected thereto high frequency current is applied to the tube to drive out any occluded gases from the metal parts and to liberate caesium from the pellet 10. The temperature of the metal parts of the device may be so controlled that the caesium is only liberated after a desired degree of vacuum is obtained. The tube is then sealed off at the point 11. The caesium when liberated cleans up the remaining gases in the tube. Enough caesium however is left in the tube to supply the desired amount of alkali for the proper operation of the discharge device.

When the metal parts of the tube become cooled the caesium is absorbed by the Aquadag and even if some alkali metal has deposited on the glass receptacle it will vaporize at room temperature and again be absorbed by the Aquadag. 105 When the tube is placed in operation, the heat from the filament will liberate the alkali and thereby supply alkali metal vapor in the space between the two electrodes.

In Fig. 2 the Aquadag is applied as a coating 110 to two metal wires or rods 12 and 13 which are secured to the filament leads 14 of a three-elemcnt tube comprising a cathode 15, grid. or control electrode 16 and anode 1'7. A caesium pellet 18 is mounted on the anode support 19 and the tube exhausted and the caesium liberated as in Fig. 1. When the tube is cooled the caesium is absorbed by the Aquadag. When the tube is thereafter placed in operation, caesium is liberated from rods 12 and 13 by heat from the filament and supplies the proper amount of alkali metal vapor in the space between the electrodes. If desired, the Aquadag may be applied directly to the anode or to the grid although the best results in a tube of this character will be obtained by applying the Aquadag to the two rods as in dicated in Fig. 2.

In Fig. 3, I have illustrated a form of electrical discharge device which is suitable for use in the space transmission of pictures. It comprises a vessel 21 filled to a desired pressure with an inert gas, such as neon, and having a pair of thin, fiat nickel or iron, plate electrodes 22 and 23 mounted on a press 24 and supplied with an electromotive force through leading-in Wires 25 and 26. The upper ends of the electrodes are spaced apart the proper distance by a cylindrical insulator 27 provided with short stiff wires 28 and 29 which engage the electrodes adjacent their upper edge. The electrodes 22 and 23 are spaced so closely that when a potential is supplied to them a discharge will take place in a well known manner between the outside surfaces of the electrodes rather than the inside surfaces. The outer surfaces of the electrodes are coated with Aquadag. Caesium or rubidium, as in Fig. l, is introduced into this tube from a pellet 30 and'is absorbed by the Aquadag when the tube is cooled to room temperature.

The energy required to separate electrons from a cathode differs with the nature of the emitting surface. Each material has a characteristic electron aiiinity. The energy required to obtain a given electron emission from a given material is dependent on the work function of the electron emitting material. This work function is a measure of the electron affinity, that is, the attraction which the electron emitting surface possesses for electrons. The starting voltage of glow tubes is, in general, dependent on the work function of the cathode, the starting voltage varying directly with the work function. A cathode, having a surface layer of caesium has a lower work function than most base metals such as, iron, nickel, aluminum, tungsten or molybdenum. Coating therefore, the surface of the electrodes with an alkali metal such as caesium, provides an electron device not only having a low cathode drop but a very low starting voltage and therefore a possibility of along life.

It is desirable in devices of this character to provide an electrode surface having a uniform coating of caesium thereon, in order to get a uniform glow over the entire outer surfaces of the electrodes since without a uniform coating of alkali metal the glow is more apt to be localized and the operating efilciency or" the tube appreciably lowered. This result is obtained by employing an Aquadag coating on the electrode, the caesium being absorbed in a smooth, even surface layer by the Aquadag. In the arrangement-disclosed the caesium is retained as a uniform thin film on the Aquadag during operation of the tube and is not heated to a temperature suiiiciently high to vaporize it.

In Fig. 4, I have disclosed a discharge device comprising a pair of hollow, nickel or iron electrodes 31 and 32, mounted on a press 33 in a container 34 the latter being filled to a desired pressure with an inert gas such as neon. The device illustrated is adapted to be employed for illumination purposes. In the construction shown the outer surfaces of the electrodes are coated with Aquadag, and a mixture of caesium or rubidium chloride and calcium placed in the hollowed out portions of the electrodes. If desired, however, caesium azide may be used to advantage as the source of the alkali because it leaves no reaction products in the tube other than caesium which will dis-color the glass. Upon application of heat, caesium or rubidium vapor is liberated and. upon cooling the device the caesium 0r rubidium is absorbed on the Aquadag coated surfaces. As in Fig. 3, the alkali metal is retained on the electrodes during operation of the lamp. In addition, to providing a low cathode drop and low starting voltage the Aquadag when employed in a lamp as disclosed, substantially prevents the deposition of caesium on the glass bulb. If any caesium is deposited on the glass when the lamp ceases to operate, it will later vaporize at room temperature and be absorbed by the Aquadag leaving the glass entirely free from any discoloration.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. electrical discharge device comprising a receptacle containing cooperating electrodes and an alkali metal, said electrodes adapted to generate alkali metal vapor during normal operation of the device, and means for positioning the alkali metal in said receptacle, said means comprising a metal member mounted in the receptacle and provided with a coating of carbon in a normally finely divided state.

2. An electrical discharge device comprising a receptacle, an alkali metal contained therein, metal electrodes mounted in the receptacle, said electrodes adapted to generate alkali metal vapor during normal operation of the device, and. carbon in a normally finely divided state on one of said electrodes for absorbing said alkali metal.

3. An electrical discharge device comprising a receptacle, containing caesium, electrodes of metal mounted therein, said electrodes adapted to generate alkali metal vapor during normal operation of the device, and a layer of carbon in a normally finely divided state on one of said electrodes for absorbing said caesium.

4. An electrical discharge device comprising a receptacle, a plurality of metal electrodes mounted therein, one of said electrodes having a coating consisting mainly of carbon in a normally finely divided state and an alkali metal, said one electrode adapted to generate alkali metal vapor during normal operation of the device.

5. An electrical discharge device comprising a receptacle, a plurality of metal electrodes mounted therein, one of said electrodes having a coating consisting mainly of carbon in a normally finely divided state and caesium, said one electrode adapted to generate caesium vapor during normal operation of the device.

6. The method of positioning an alkali metal in an electrical discharge device, which comprises coating a portion of said device with a colloidal suspension of graphite and water, liberating alkali metal in the device and cooling the device to thereby permit the alkali metal to be absorbed by said coated portion.

7. An electrical discharge device comprising a ing a coating consisting mainly of carbon in a receptacle, a filling of rare gas therein, a pair of normally finely divided state and alkali metal, metal electrodes mounted in the receptacle, and adapted to generate alkali metal vapor durmeans whereby an eleotro-motive force may be ing normal operation of the device.

5 supplied to said electrodes, said electrodes hav- ERNEST E. CHARLTON. 80 

